Here is a Slim theme I made by editing an existing theme called archlinux-darch-grey. It is basically a black background with the Antergos logo and name with a log in box under it.
Just unzip and put it in /usr/share/slim/themes. If you already have the theme archlinux-darch-grey this will replace it. Make sure you edit your /etc/slim/conf to the new theme name so it displays.